This invention relates to the field of CRT devices in data display terminals and, more particularly, to the provision of improved resolution even in the outermost areas of the CRT display.
Since the screens of most cathode ray tubes are as nearly flat as it is economical to make them, a problem arises in causing the electron beam to be sharply focused at all points of the screen. The problem arises because the beam must travel varying distances to the various points on the screen, the variation being a function of the distance from the longitudinal axis of the screen to the given point. The most common solution is to set the static (DC) focus voltage at a level which will provide optimum focusing at some point intermediate the center of the screen and the farthest away points with minimum defocusing at both the center and the corners. Also, schemes have been proposed for adding varying voltages to the static focusing voltage. One such scheme adds to the static voltage the sum of one parabolic waveform at horizontal line rate and one at vertical frame rate. This scheme is disclosed in a copending U.S. patent application Ser. No. 908,297, assigned to the same assignee as is the present invention. The scheme provides greatly improved resolution over the entire CRT screen, however, with the increased use of CRT devices as in a display terminal where the entire screen area is used for the display of relatively small alphanumeric characters, a need is felt for still more resolution, particularly in the far corners of the screen.